If every mare in a herd produced a foal, you’d see birth rates approaching, possibly exceeding, 50% per year.
Mares tend to outnumber stallions on the range, indicated by data from roundups.
A 45/55 split between males and females would not be unusual.
The Montana Solution magnifies the disparity, even taking it outside the range of expected variation from a random process.
Foals typically represent fifteen to twenty percent of the horses captured in a roundup.
Not all mares want to be bred, at least in your host’s experience.
They resist the stallion’s advances.
You may have heard about horses taking their riders under trees to knock them off.
Same thing happens at the ranch. The mare, untreated, goes straight for the barn when the colt, uncut, tries to mount her, knocking him off. This has been going on for several weeks, complete with dust and flying gravel.
This suggests that there may be other ways of letting the horses manage themselves on the range, without the interference of humans. They may know more about appropriate herd sizes, genetic diversity and the risks of inbreeding than we think.
“Dart them anyway, it’s good for business.”
Western Horse Watchers Association 